The activated sludge wastewater treatment process is the most common technology used today and solids separation is a critical and often problematic component of this technology. This leads to requirements for larger clarifiers, increased pumping costs of the diluted settled solids stream, and increased costs for solids treatment. For the past thirty years, the majority of the research in secondary settling problems has focused on the role of filamentous bacteria, and also factors such as floc size and shape. The effects of variable biomass density on settleability of biosolids produced in full scale activated sludge wastewater treatment systems has been recently been demonstrated, and seasonal variability in biomass settleability is known to occur at many wastewater treatment plants. The role of density in such temporal variations has not been previously studied.
Research evaluating temporal variation in biomass density and filamentous bacteria content and the roles these parameters play in solids separation was performed. Three different filamentous bacteria quantification methods were employed and comparison of these methods was evaluated. Additionally, the effects of metal coagulant addition for phosphorus removal on biomass density and settleability were evaluated.
The objectives of this research were to (1) evaluate whether biomass density varies on a seasonal basis in full scale wastewater treatment systems, and if this contributes to seasonal variations in settleability, (2) evaluate if biomass density varies on a shorter timescale (weekly), and whether this is correlated with changes in settleability, (3) compare various methods of filamentous bacteria quantification, (4) investigate how precipitant addition effects biomass density and settleability.
Seasonal variation in biomass density was observed in four activated sludge plants, with high density occurring during warm weather and low density occurring during cold weather. Biomass density and filament content were both determined to influence settleability, and non-volatile solids content was strongly correlated with biomass density. Weekly variations in biomass density were not observed in selector equipped wastewater plants under stable operating conditions; however, significant variation in biomass density, settleability, and polyphosphate content were observed during process start-up and process upset events at a single full scale wastewater plant designed to perform nitrification, denitrification, and enhanced biological phosphorus removal.
Methods of quantifying filament content based on microscopic imaging were compared, including the Filament Index ranking, automated image analysis, and manual tracing methods. The filament length/floc area ratio determined by automated and manual methods is a measure that is reliant only on information in microscope images. Filament length/dry solids were determined using a hemacytometer slide and manual tracing, which allows quantification of filaments within a known volume. The relationships between these three methods of measurements were determined, which may aid in comparisons of filament content presented in other studies, both past and future.
Precipitant addition using aluminum and iron salts was positively correlated with biomass density; however, counter to the study hypothesis, settleability degraded rather than improved. This result may be due to the development of large precipitate structures that, although of relatively high density, inhibited compaction of the biomass.